We are in the midst of worldwide epidemics of obesity and diabetes. According to the World Health Organization, over 1 billion adults (~15% of the world population) are either overweight (body mass index (BMI) >25) or obese (BMI>30) and more than 150 million adults have diabetes, most of which is type 2 diabetes driven by the insulin resistance associated with obesity. There are at least two clearly distinguishable forms of fat: white adipose tissue (WAT), which stores energy, and brown adipose tissue (BAT), which burns energy for thermogenesis. Thus, how fat an individual is not just a reflection of how much s/he eats, but also how much WAT vs. BAT might be present and how active the BAT might be. Methods for evaluating the amount and activity of BAT are crucial for evaluating strategies to increase BAT thermogenesis in order to reduce obesity. We propose to design and test novel RNA molecules that will sense the presence of UCP1 mRNA, a marker for the presence and activity of BAT. These novel RNAs will be part of a riboswitch that, when the biomarker is present, activates an associated aptamer, that will bind a radioactive molecule, 123I thyroid hormone, which can be localized and quantitated with SPECT/CT. We shall investigate delivery of this riboswitch into BAT cells in culture, using a novel cell based SELEX approach. SPECT/CT studies will enable us to image and quantitate the uptake and retention of a radioactive ligand responsive to the presence of UCP1 mRNA in BAT tissue in mice. By combining the Hebrew University Medical School PI's experience in the field of RNA binding, our consultant/collaborators' expertise in aptamer selection and internalization of cell surface markers, the Joslin PI's experience in BAT physiology and BAT tissue culture, and the BIDMC PI's expertise in nuclear medicine, this project proposes, to the best of our knowledge, a completely novel approach to specifically target and quantitate UCP1 mRNA, a key marker for the presence and activity of BAT.